1. Field of the Invention
The present invention relates to a radiation thermometer for measuring the temperature of an object with an infrared light beam and more specifically to an optical system therefor.
2. Description of the Prior Art
A radiation thermometer has usually been employed for measuring the temperature of a measuring object at a position displaced therefrom. Therefore the measuring range of the thermometer is comparatively small. Moreover, since it is essential for an operator to accurately recognize the measured temperature for the measuring object, a finder is provided. An example of an optical system of such a radiation thermometer is shown in FIG. 1. In FIG. 1, a light bundle used for temperature measurement is respectively reflected by a main mirror (2) and a sub-mirror (4) and enters a light sensing element (6) and thereby a temperature can be measured in accordance with the infrared light beam component received by element (6). Meanwhile, a light bundle for the finder used for monitoring a measuring object passes a finder lens (8) provided in front of the sub-mirror (4), is guided to the outside of measuring light bundle by a mirror (10), is reflected again by a mirror (12) and is finally guided to a finder lens system (14). A measuring mark plate, having a mark for displaying the measuring object area within the visual field of the finder, is indicated as numeral (16). Accordingly, the measuring object area is displayed by said mark within the visual field of finder in accordance with the light sensing area of the light sensing element (6) for recognition of measuring object.
In the prior art described above, the optical system for measurement consisting of the main mirror (2), sub-mirror (4) and light sensing element (6), and the optical system for monitoring consisting of the finder lens (8), mirrors (10), (12) and finder lens system (14) respectively have independent optical axes. Therefore adjustment is necessary to coincide these two optical axes and it is difficult to form a compact system as a whole. Particularly, deviation may be generated in both optical axes due to the aging or a change in temperature and moreover a very expensive mechanism is also required because any deviation of a optical axis must be eliminated even in case each element is moved along the optical axis for a the focusing.